In fluid mechanics, a boundary layer is developed by viscous effects in the region immediately adjacent to a bounding surface and it also causes the surface friction which is related to the drag. Boundary layer separation occurs under adverse or positive pressure gradients when the portion of the boundary layer closest to the wall departs from the surface (Simpson, 1989, 1996). This breakdown of the boundary-layer flow is exhibited as a thicker more turbulent region of low wall shear stress that produces a significant modification of the pressure field from the attached boundary layer condition and mean or time-averaged flow reversal in some instances. Therefore, boundary layer separation results in a large increase in the pressure drag on the body, which is most of the total drag, and an increase in related acoustic noise (Simpson, 1989; Lin, 2002).
In hydraulic engineering, a scoured bed around the hydraulic structure is most often a consequence of separation of the incoming boundary layer as it encounters the hydraulic structure and the resulting vortical flow. The scour of sediment around the base of a hydraulic structure is a major cause of catastrophic bridge collapse. Therefore, flow separation control techniques around the hydraulic structure can be effective to prevent flows that cause scour.
The majority of the heat transfer to and from a body also takes place within the heat exchanging fluid boundary layer. The low momentum region developed next to the flow separation results in very poor heat exchange performance between the body surface and the flow. Therefore, suppressing the boundary layer separation increases the rate of heat exchange between a body surface and a heat exchanging fluid.
There are a number of passive and active ways to control boundary layer separation, such as vortex generators, boundary layer trips (turbulators), suction and ejection devices, etc. In the discussion to follow, the method of separation control via vortex generators is described in terms of the current state-of-the-art.